Storage devices having a mechanism in which data is written on a rotating storage medium via a writing head are used as nonvolatile large-capacity storage devices for various kinds of applications. Typical examples of such storage devices include magnetic disk devices. Also, optical disk devices, magneto-optical disk devices, etc., are classified into such a category. Such a storage device is capable of writing serial data at a considerably high data writing rate. However, such a storage device requires a waiting period for mechanically moving a writing head to a specified region. This leads to a problem of a slow random access rate. In particular, such a storage device has a problem in that, in some cases, the access time from the execution of a writing command up to the start of a writing operation is significantly delayed, depending upon the position at which the writing head was located immediately before the execution of the writing command.
Compared to rotating storage devices, nonvolatile memory devices such as flash memory have the advantage of a high random access rate, and the advantage of small irregularities in access time depending upon the region (address). In recent years, the capacities of such nonvolatile memory devices have increased dramatically, and they have begun to be used as a substitute for rotating storage devices in some fields of application. However, nonvolatile memory has a problem in that there is a limit to the number of times data can be repeatedly written to the same storage region. For example, nonvolatile memory has the disadvantage that writing data one hundred thousand times or more deteriorates the storage performance of a storage cell. In order to compensate for this disadvantage, a combination of a rotating storage device (unit) and a nonvolatile memory device has been proposed as discussed in Japanese Laid-open Patent Publication No. 2007-115232, Japanese Laid-open Patent Publication No. H07-84880, Japanese Laid-open Patent Publication No. H10-154101, and Japanese Laid-open Patent Publication No. 2003-167781.